1. Field of the Invention
This invention relates to an improvement in cameras for exposing photosensitive materials for producing half tone or similar images and in particular to an improved structure for "flash" exposing the photosensitive materials.
2. Description of the Prior Art
Previous to the present invention photosensitive printing plate material having half tone quality could be exposed by using a projection type glass screen with opaque ruled lines. For a given screen and aperture the optimum distance for the screen from the receptor material was calculated to produce the properly formed half tone dots. Once a main exposure was achieved, a "flash" exposure using an all white overlay on the copy board was used to reexpose the plate to lighten the photograph's darkest areas. This effectively reduced the overall contrast of the half tone copy and extends the screen range.
Other attempts in the prior art to provide the flash exposure without using the white overlay on the copy board have been attempted. One attempt was to use a lamp positioned off the optical path or, to balance illumination, by using multiple bulbs spaced symmetrically about the projection lens. In each of these instances the result was that the quality of the dots was compromised. When the path of the light used for flash exposure from the source to the photoplane follows a different path through the screen than that of the originally projected light, the dot quality is affected.
The use of an all white overlay on the copy board, such that the flash exposure is through the same lens and along the same optical path, increases the time spent by the camera operator when this time could be better spent placing the next image to be photographed on the copy board.
The present invention therefore has provided a method for obtaining flash exposure to improve half tone reproduction and at the same time to reduce the time necessary in making each of the exposures without compromising the quality.
The present invention has afforded a means for effectively centering the flash illumination along the optical path and inside the bellows or light-tight enclosure. The present exposure structure is so located that it moves with the lens when changing magnification ratios and it is out of the way during the main exposure of the image from the copy board. Further, it allows daylight camera operation.